This invention relates generally to systems and methods for restricting movement in ducts, and, more generally, to methods and systems for increasing the natural frequency of a duct.
Exhaust systems for aircraft or other vehicles may have features that mix hot exhaust air and cooler ambient air to limit temperatures of the surrounding vehicle structure or to limit the heat signature of the vehicle. One method of achieving cooler exhaust temperatures is to attach a flow mixer to the outlet of the exhaust system. At least some known flow mixers include a plurality of lobes, or “flutes,” that promote mixing of the exhaust air and cooler ambient air.
A potential drawback is that the stiffness of the flow mixer may be reduced because of the lobed or fluted designs. Consequently, at engine or rotor frequencies that align with the flow mixer's natural frequency, such alignment of natural frequencies (resonance) may lead to increased deflection and stresses that may limit or prevent use of a fluted design, or may require use of thicker, heavier duct walls. Further, static and/or dynamic pressures on the walls of the flutes may cause large deflections in the area of the flutes and may impact flow mixing.
Duct size and configuration of an exhaust system may also contribute to natural frequency. At least some known exhaust systems use ducts having relatively small flow mixers in an attempt to avoid certain frequencies without additional stiffening features. However, such relatively low-flow mixers may not provide optimal efficient mixing of hot exhaust air and cooler ambient air. Furthermore, at least some exhaust systems may use multiple small, relatively low-flow mixers to increase the mixing efficiency. However, multiple flow mixers may increase manufacturing costs, maintenance costs, and/or weight associated with the exhaust system.